Right now, there is no versioning of the OpenRaster standard. The wiki keeps track of the revisions, but additions and changes are done without nothing in the document itself when the change or addition was done.

Keeping track of revisions and contributions using wiki-software provided versioning and usernames should be considered actively harmful to the health of the specification. Case in point: the wiki home of the spec has moved recently, and revision history has been lost. Sorry about that. I tried to annotate in places, but a lot was lost to moves back in the CREATE wiki days :(
During my importing of the old CREATE wiki stuff, I namespaced the specification-type information under http://www.freedesktop.org/wiki/Specifications/OpenRaster/Draft considering that it might be used as a place for drafting and refining the "next version" of the spec ... whatever that means. We could get more formal than this by either creating a Specifications/OpenRaster/<version> set of pages for each bump as some projects do, or by working on it in git as you suggest. Technically I'd be happy to do either, but IMO we should use the wiki we have at first until the draft is ready for a formal version number. It isn't right now; there are several omissions and some core pages are still a bit of mish-mash of normative stuff and un-thought-through proposals.
Hopefully we can get a minimal 0.1 created sometime soon. 1.0 before the end of the year? :D